In the field of video and image processing, it may be necessary to apply horizontal and/or vertical scaling to images. For example, certain format conversion required by standards, such as ATSC (Advanced Television Systems Committee) and certain applications such as video zooming in, zooming out, and picture-in-picture (PIP) functions, require vertical and/or horizontal scaling.
FIG. 1 illustrates a conventional image scaling system. The conventional image scaling system 100 may comprise an upsampler 101, a lowpass filter (LPF) 103, and a downsampler 105. The upsampler 101 has an upsampling factor of I, and the downsampler 105 has a downsampling factor of D. The source/destination scale factor, therefore, is D/I.
During operation of the conventional image scaling system 100, an incoming video signal x(n) may be upsampled by the upsampler 101 utilizing an upsampling factor of I. The initial sampling rate of the video signal x(n) may be F, which may indicate the number of received pixel samples per second. During upsampling of the video signal x(n), an (I−1) number of zeros may be inserted between each two received pixel samples. Upsampling may result in passband copies of X(ω), which is the Fourier Transform of x(n) in the frequency domain, at every multiple of
            2      ⁢      π        I    .Low Pass Filtering (LPF) should be applied to reject the frequency components beyond
      2    ⁢    π    Iand thus would compress the bandwidth of the signal by a factor of I. Downsampling expands the spectrum of the signal by a factor of D. The downsampling factor D may indicate that one pixel sample is retained for every D pixel samples. If the downsampling factor D is greater than the upsampling factor I, aliasing may occur when copies of X(ωD/I) overlap, so additional LPF 103 may be used after the upsampler 101 to isolate the baseband copy of X(ωD/I).
The LPF 103 may comprise a finite impulse response (FIR) filter, which has the advantage of having linear phase and being easy to implement. For example, a certain system may employ an 8-phase-by-8-tap FIR filter and a 4-phase-by-4-tap FIR filter to achieve the horizontal and vertical scaling, respectively. While such architecture accomplishes satisfactory results for some video format conversions, aliasing may be present during the video scaling process and may become very severe and objectionable for large scale factors for downscaling such as those larger than 3:1 vertically and 6:1 horizontally, which may be due to limited number of taps utilized in the FIR filter.
One solution to such a problem would be to increase the number of taps. However, increasing the number of taps in the FIR filter may become costly and may not contribute substantially to resolving the aliasing problem. For example, if the number of taps in the vertical FIR filter is doubled to 8-taps, the cost increases, but the filter may not achieve 10:1 downscaling well. As a result, the cost is substantially increased in return for slight improvement in performance.
FIR filtering may require very high order and long tap-length filters when very low cutoff frequency is needed, which may result in a significantly high cost for hardware implementation. Due to cost restrictions, most systems may not afford to increase the size of the filter arbitrarily to achieve a desirable downscaling factor. In the above example, with the 8-phase-by-8-tap FIR horizontal filter and a 4-phase-by-4-tap FIR vertical filter, aliasing starts to manifest and may become very objectionable when the scale factor for downscaling is larger than 3:1 vertically and 6:1 horizontally, for example.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.